The present invention relates to a rotation control device for an information recorded disc which is applicable to a video disc player or the like. The player rotates a disc with recorded information and sync signals at a predetermined speed.
Today, such discs are available for high density recording and playback of information signals having in a wide frequency range. When used for recording and playback purpose, the discs simplify a drive mechanism therefor compared to other forms of information recording media, and thereby renders the construction of a playback apparatus simple and economical. Various kinds of recording/playback apparatuses have been put to practical use utilizing such features of discs.
The primary requisite for a playback apparatus for discs is that it is capable of controlling the rotation of a disc to a speed and a phase which are preselected for faithful reproduction of information signals out of the disc.
A prior art disc rotation control device generally comprises a motor for driving a disc which is fixed in place on a turntable by a clamp, a playback or read element for reading an information signal out of the disc, a signal processing circuit for processing the information signal, a phase comparator for generating a first difference signal by comparing a sync signal extracted from the disc by the signal processor and a reference signal generated by a reference signal generator, a frequency generator for generating a signal whose frequency corresponds to a motor rotation speed, a frequency-to-voltage (FV) converter for transforming a signal output from the frequency generator into a voltage, an operation circuit for generating a second difference signal by comparing a voltage output from the FV converter with a reference voltage and adding the second difference signal to the first, a phase compensator for compensating the phase of a signal output from the operation circuit, and a motor drive for driving the motor by applying thereto a drive current which corresponds to the phase compensated signal.
The prior art control device described above includes a first control loop for speed control and a second control loop for phase synchronization between a sync signal recorded in a disc and a reference signal output from the reference signal generator. The first control loop is completed by the motor, frequency generator, converter, operator, phase compensator, motor driver which are connected in this order. The second control loop, on the other hand, is completed by the motor, read element, signal processor, phase comparator, operator, phase compensator, and motor drive which are also connected in this order. At a start-up of the motor, a speed control is performed by the first control loop such that the rotation of the disc is quickly accelerated to a predetermined speed. This is followed by a phase control performed by the second control loop so that the rotation phase of the motor is locked to the phase of the reference signal which is generated by the reference signal generator. The two sequential controls allow a disc to rotate at a specific speed which correctly reproduces an information signal out of the disc.
In the start-up stage of the motor operation, the rotation speed progressively increases from zero and, therefore, the relative reading velocity of the disc and read element progressively increases from zero. For this reason, a demodulator which is installed in the signal processor to demodulate information signals read out of a disc does not produce any information signal or sync signal unless the disc rotation is accelerated substantially to a predetermined speed. This means that in this operation, the speed control by the first control loop occurs but not by the second control loop.
The reason why the demodulator produces no information signal or sync signal except for a predetermined disc rotation speed is as follows. While signals recorded in discs are FM waves or the like, the demodulator for demodulating them is unable to produce demodulated outputs except for signals which fall in a specific frequency range. In other words, unless the disc rotation speed becomes close to predetermined one, the frequencies of FM waves corresponding to information and sync signals fall out of the demodulator's operational frequency range.
As soon as the disc rotation is controlled by the first control loop substantially to the predetermined speed, the signal processor starts producing demodulated information and sync signals. The phase comparator compares the phase of the sync signal output from the signal processor with that of the reference signal output from the reference signal generator, delivering a difference signal to the operation circuit or operator. Where the frequency difference between the two signals input to the phase comparator is larger than one half the cut-off frequency of the second control loop, the phase comparator produces an alternating difference signal whose repetition frequency is the frequency difference between the two signals. However, due to a relatively low cut-off frequency of the second control loop, the system fails to respond to the difference voltage so that a zero or non-controlled state occurs preventing the phase from being locked for the predetermined speed.
The problem discussed above will not arise insofar as the velocity control performed by the first control loop is accurate enough to drive a disc exactly at a predetermined speed. Nevertheless, the accuracy of motor speed control by the first control loop depends upon the accuracy and stability of the FV converter which is included in the first control loop. The operation point of the FV converter is generally dependent upon a time constant determined by a resistor and a capacitor and, therefore, the accuracy of the FV converter is affected by the accuracy of the resistor and capacitor.
The accuracy and stability of resistors and capacitors cannot be increased beyond a certain limit. Hence, where the frequency of the signal output from the reference signal generator is relatively high, difficulty has been experienced in enhancing the accuracy of speed control by the first control loop to such a degree that it is confined to within the frequency difference which allows the second control loop to perform pull-in, that is, within a frequency difference which is not smaller than one half the cut-off frequency of the second control loop.